His theory of the arts in their reciprocal relations and affinities throws interesting light upon the qualities of his own genius and his method in practice.

"The science of design, or of line-drawing, if you like to use this term, is the source and very essence of painting, sculpture, architecture, and of every form of representation, as well too as of all the sciences.

He who has made himself a master in this art possesses a great treasure.

Sometimes, when I meditate upon these topics, it seems to me that I can discover but one art or science, which is design, and that all the works of the human brain and hand are either design itself or a branch of that art." This theme he develops at some length, showing how a complete mastery of drawing is necessary not only to the plastic arts of painting and sculpture, but also to the constructive and mechanical arts of architecture, fortification, gun-foundry, and so forth, applying the same principle to the minutest industries.
With regard to the personal endowments of the artist, he maintained that "a lofty style, grave and decorous, was essential to great work.
Few artists understand this, and endeavour to appropriate these qualities.

Consequently we find many members of the confraternity who are only artists in name.